firmware-utils: mksercommfw: overhaul image creation

Move the zip compression into a build recipe. Pad the image using the
existing build recipes as well to remove duplicate functionality

Change the code to append header and footer in two steps. Allow to use a
fixed filename as the netgear update image does.

Use a fixed timestamp within the zip archive to make the images
reproducible.

Due to the changes we are now compatible to the gnu89 c standard used by
default on the buildbots and we don't need to force a more recent
standard anymore.

Beside all changes, the footer still looks wrong in compare to the
netgear update image.

Signed-off-by: Mathias Kresin <dev@kresin.me>

include/image-commands.mk

@@ -168,6 +168,16 @@ define Build/gzip
 	@mv $@.new $@
 endef
 
+define Build/zip
+	mkdir $@.tmp
+	mv $@ $@.tmp/$(1)
+
+	zip -j -X \
+		$(if $(SOURCE_DATE_EPOCH),--mtime="$(SOURCE_DATE_EPOCH)") \
+		$@ $@.tmp/$(if $(1),$(1),$@)
+	rm -rf $@.tmp
+endef
+
 define Build/jffs2
 	rm -rf $(KDIR_TMP)/$(DEVICE_NAME)/jffs2 && \
 		mkdir -p $(KDIR_TMP)/$(DEVICE_NAME)/jffs2/$$(dirname $(1)) && \